Various backlash-free planetary gear units are known from the prior art, e.g. from US 2003/0073537 A1 and from DE 197 57 433 A1.
US 2003/0073537 A1 discloses four different embodiments of a backlash-free gear unit. A planetary gear unit has here an internal gear which is split into two parts with different diameters. As is normally the case with planetary gear units, toothed planet gears mesh with a sun gear on the one hand and an internal gear on the other. In the case of this US application, two parts of the respective planet gears mesh with the two parts of the internal gear. The planet gears are divided into two planet subgears, one of said planet subgears having a diameter which is smaller than that of the other.
The planet subgears having the smaller diameter mesh with the internal gear which is smaller in diameter than the other internal subgear. The larger internal subgear is in mesh with the planet subgears having a larger diameter than the other planet subgears. One of said internal subgears is positioned stationary, whereas the other internal subgear is freely rotatable. The respective two planet subgears defining an entire planet gear are freely rotatable relative to one another in opposite directions of rotation about one and the same planetary axis of rotation. The teeth of one of said planet subgears can therefore be rotatably displaced relative to the teeth of the other planet subgear to a minor extent, so that the respective teeth of the planet subgears, which are in mesh with the respective internal gears, will abut on the respective tooth flanks of the internal subgears without backlash.
In the four embodiments disclosed, different ways of accomplishing the preload, i.e. for accomplishing the rotary displacement between one planet subgear and the associated other planet subgear, are disclosed.
The first embodiment described is so conceived that a pin protrudes through an opening of the first planet subgear up to and into the second planet subgear. The pin is fixedly anchored in the second planet subgear. The upper end of the pin is in contact with a clip, said clip having a partially annular cross-section and being connected to a second pin which is fixedly anchored in the planet subgear. By means of this pin/clip combination a preload can be achieved between the planet subgears.
In a second embodiment, the preload can be accomplished by means of a conventional helical compression spring, which is arranged in a slot of the first planet subgear and which is in contact with the first planet subgear and a pin, said pin being fixedly anchored in the second planet subgear.
In a third embodiment, a concentrically arranged, torsionally flexible shaft is disclosed. When the first planet subgear is fixed to the shaft in such a way that the teeth of the two planet subgears are not in alignment, a torsional force will act on the two planet subgears, when the teeth of these two planet subgears are arranged in alignment with one another.
Alternatively to the above embodiments, a fourth embodiment of US 2003/0073537 A1 is so conceived that a radially arranged flexure element for applying the preload between the two planet subgears is in contact with a shaft and a pin which is arranged eccentrically parallel to the axis of rotation of the two planet subgears.
The prior art also comprises various documents which disclose that a preload is applied to gears used in normal gear units. EP 1 034 923 A1, for example, discloses spring elements for avoiding gear backlash in gear units. Also U.S. Pat. No. 2,845,809 deals with the fact that two gears are preloaded relative to one another so as to prevent backlash. Also JP 02113154, U.S. Pat. No. 3,138,035 as well as U.S. Pat. No. 5,492,029 disclose solutions for avoiding backlash between gears. Related prior art is also known from DE 7400276 U, US 2004/0154422 A1 and FR 2805327 A1.
The solutions known for conventional gears are disadvantageous insofar as such gear units require a lot of space. Especially for the drive units of small electric appliances, it is important that the space required is only small.
In the case of planetary gear units it is very complicated to eliminate backlash. The four solution variants according to US 2003/0073537 A1, for example, have a lot of drawbacks. It is e.g. necessary that one of the internal subgears is arranged in a stationary manner and that the planet subgears have different diameters. Also the preloading of the planet subgears relative to one another is accomplished in a rather complicated manner: the variant of using a soft torsional spring having a partially annular cross-section for accomplishing the preload necessitates that the respective pins project in the direction of the planetary axis of rotation so as to prevent the clip from falling off and the resultant failure of the planetary gear unit. Additional space will thus be needed.
When a helical compression spring which is arranged orthogonally to the planetary axis of rotation is used, the spring will have to be additionally protected against falling out of the opening of the first planet subgear, and also this will increase the amount of space required.
Also the third variant, which makes use of a torsionally flexible shaft for preloading, requires complicated fixing solutions and makes mounting particularly laborious in view of the small dimensions in planetary gear units, especially in view of the small degrees of freedom. Also the variant concerning the use of a radially arranged preloading element requires a substantial amount of space.